The development of multicellular organisms is highly dependent upon cell- cell communication and signal transduction pathways. Many such pathways are mediated by receptor tyrosine kinases that are activated by the binding of spatially restricted ligand. The failure of these signalling processes can result in highly disrupted development, and in some cases, ectopic or constitutive signalling can be oncogenic. The development of the terminal regions of the Drosophila embryo provides an excellent model system in which to investigate the genetic and molecular aspects of the spatial regulation of receptor activation. This pathway is mediated by torso, a receptor tyrosine kinase that is present throughout the embryonic membrane but activated only at the ends of the embryo. The spatial information that localizes the torso signal appears to be generated in the somatic follicle layer that surrounds the developing oocyte and to be regulated through the expression of the gene torsolike by specialized polar subpopulations within the follicular epithelium. The overall goal of this proposal is to elucidate the mechanisms by which spatial information is generated within and transmitted by the polar follicle cells to the future embryo. To investigate the role played by torsolike, antibodies will be generated against the torsolike product and used to analyze its subcellular distribution. To determine the function of the polar follicle cells in the patterning of both the embryo and the follicle, conditionally toxic gene products will be specifically expressed in these subpopulations. To dissect the pathway by which polar identity is established in the follicle cell layer, mutations that affect the development of the polar follicle cells will be identified and characterized. Several lines of evidence suggest that many genes involved in Drosophila oogenesis are also required at earlier times in development and are consequently lethal to organism when mutated. To isolate mutations in such genes, a novel mutagenesis screen will be carried out that will utilize techniques that permit the analysis of phenotypes in females with mosaic ovaries that contain clones of homozygous mutant cells.